Effect of tetrahydroquinoline dyes structure on the performance of organic dye-sensitized solar cells

Eleven novel donor acceptor pi-conjugated (D-pi-A) organic dyes have been engineered and synthesized as sensitizers for the application in dye-sensitized solar cells (DSSCs). The electron-donating moieties are substituted tetrahydroquinoline, and the electron-withdrawing parts are cyanoacrylic acid group or cyanovinylphosphonic acid group. Different lengths of thiophene-containing conjugation moieties (thienyl, thienylvinyl, and dithieno[3,2-b;2',3'-d]thienyl) are introduced to the molecules and serve as electron spacers. Detailed investigation on the relationship between the dye structure, photophysical and photoelectrochemical properties, and performance of DSSCs is described here. The bathochromic shift and increase of the molar extinction coefficient of the absorption spectrum are achieved by introduction of larger conjugation moiety. Even small structural changes of dyes result in significant changes in redox energies and adsorption manner of the dyes on TiO2 surface, affecting dramatically the performance of DSSCs based on these dyes. The higher performances are obtained by DSSCs based on the rigid dye molecules, C2 series dyes (Figure 1), although these dyes have lower light absorption abilities relative to other dyes. A maximum solar-to-electrical energy conversion efficiency (eta) of 4.53% is achieved under simulated AM 1.5 irradiation (100 mW/cm(2)) with a DSSC based on C2-2 dye (V-oc = 597 mV, J(sc) = 12.00 mA/cm(2), ff = 0.63). Density functional theory (DFT) calculations have been performed on the dyes, and the results show that electron distribution from the whole molecules to the anchoring moieties occurred during the HOMO-LUMO excitation. The cyanoacrylic acid groups or cyanovinylphosphonic acid group are essentially coplanar with respect to the thiophene units, reflecting the strong conjugation across the thiophene-anchoring groups.